"Okay, to avoid what you'd call motion sickness, you need to tighten your entire lower body--every muscle. Calves. Thighs. Glutes. Create a dam, right at your waist. Block off the flow of blood below your hips to prevent it from pooling down there. Check?" Check. My ass puckers. One does not take lightly an order from Maj. Lance Annicelli, U.S.A.F., Chief, Altitude and Acceleration Operations, Brooks City Air Base, San Antonio, Texas.

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Annicelli is crew cut, menacingly handsome, with a vague resemblance to the actor Viggo Mortensen. He's explaining that the cardiovascular system is the primary component of the body affected by g-forces. The human skeleton and soft tissues, he says, can withstand such stress. Up to a point. The circulatory system, however, consists of elastic blood vessels that need a well-defined pressure and volume to function. Excessive g's disrupt circulatory flow.

Now Annicelli points to the whiteboard in the ready room. A diagram, his sketch, of my upper torso's normal bloodflow--heart to brain to eyes. As I approach 3 g's, my heart will be unable to pump suffi-cient blood to my brain, which will reduce my systolic arterial blood pressure. As the blood supply drains from my head, the drop in intraocular pressure will collapse my retinal arteries. A form of temporary blindness or tunnel vision will ensue. Gray-out, he calls it. Worse than motion sickness.

"Now, to breathe. Your weight will increase onefold for every g you pull. We're taking you up to 5 today. What do you weigh?"

"One-ninety."

"That means when you're in the centrifuge, your body weight will be nearly a thousand pounds. It'll feel as if a giant boulder is sitting on your chest. Your brain holds enough oxygen reserve to last around 6, 7 seconds. Beyond that, you need to replenish. But your internal organs will be dragging down your diaphragm. This disturbs your respiration mechanics. And if you exhale too deeply, you won't have the strength to get any air back in. Your rib cage will be too heavy. Understood?"

Understood.

"I want you to inhale profoundly. Hold it in. Close the airflow at your thorax. Your breaths will come in short microbursts. Like this." He demonstrates how to breathe "up the g ladder." You use the word "hook." Start with a deep inhalation and bear down against the glottis. This is the "hoo" in the word "hook."

"Yes, sir." He leads me down the stairs to the centrifuge. How had it come to this? I'd merely been trying to learn why so many people, myself included, become nauseated on airplanes and when trying to read in moving cars.

During the first World War, pilots complained of a peculiar phenomenon. "Fainting in the air," they called it. Six-, 7-second blackouts; up to 20 seconds of incapacitation before they regained their senses. It occurred most often during aerobatic maneuvers, like pulling out of high-speed dives or banking into tight turns during dogfights. Alarmed, scientists and flight surgeons deduced that decreased bloodflow to the brain and environmental stress on the vestibular system, or the nonacoustic portion of the inner ear, were the root causes. This stress--the rate of change in direction or velocity--is merely the extreme form of the air, sea, and motion sickness most of us have experienced in our modern, fast-transport society.

The vestibular system is nature's way of maintaining balance in a healthy human body. What our eyes are seeing, the inner ear is sensing and the muscles are feeling. But when our sensors go awry, motion sickness ensues. It's not an illness per se, but a group of symptoms--nausea, vomiting, pallor, cold sweats--best defined as a normal reaction to an abnormal physical situation: your sweaty palms during flight turbulence; the shade of green you turn watching waves from a deck of a moving ship. Even tilting your head on a languid merry-go-round will trigger the false sensation of moving in a direction you are not.

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Multiply these physical perceptions tenfold and you have some idea of the stress faced by fighter pilots acclimating to rapid changes in direction at multiple g's. As aircraft became more powerful and maneuverable over the past century, medical researchers determined that technology had outpaced the constitution of the mammalian body. So in 1964 the air force installed a human centrifuge at what is now Brooks City Air Base in San Antonio. Begun as a pilot-training headquarters, Brooks evolved into a superlab for aerospace medicine. Its centrifuge integrates support equipment to monitor pulmonary activity, blood-gas levels, and cardiovascular reaction to extreme g-forces.

In other words, it's where fighter pilots and astronauts learn how to avoid puking.

A tight spot

"Locked and loaded. Okay. Concentrate on the red light straight ahead and the two green peripherals. Grab your hand brake. Remember to keep it squeezed unless you want to come down."

Technical Sgt. Kevin Johnson's voice over the intercom has a soothing effect. There's nothing in his dulcet tones to suggest that my body is about to implode. I'm harnessed in the windowless cockpit of the Brooks City base's centrifuge, a cramped gondola about the size of the driver's side of a Volkswagen Beetle.

My seat, procured from an F-16 fighter jet, is suspended at the end of a 20-foot arm powered by four 250-hp motors that will lift, rotate, and spin the gondola at about 45 mph. To my vestibular system, the carnival ride from hell. Electrode wires to monitor my heart protrude from my chest. About 10 feet above, invisible to me in a glass control booth, a flight surgeon, an aerospace physiologist (Annicelli), a central observer, a data specialist, and Johnson, the centrifuge's operator, study me through two closed-circuit cameras. The hand brake Johnson refers to is a dead man's brake.

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If I lose consciousness and release my grip, the centrifuge will automatically decelerate.

"You ready?" asks Johnson.

"Ready."

"Okay. Three, two, one, engage."

A small jolt. The centrifuge is capable of achieving 6 g's per second. But on this, my first 50-second run, they take me up gradually at 0.1 g per second. Johnson reminds me to begin my lower-body g-strain as soon as I start to lose my peripheral vision. When the two green lightbulbs at either side of my forehead disappear and the red bulb directly before me begins to dim, he says, time to breathe.

"Drink some water," says Johnson. "There's a bottle of it right at your feet. If you're feeling something like motion sickness and have to puke, there's a bag down there, too. Or use the inside of your flight suit. And, um, you can let go of the hand brake now."

The odd thing, explains Col. (Dr.) Richard DeLorenzo, research flight surgeon at Brooks, is that I'm precisely the wrong physical type to pull any kind of serious g's--long, lean, and lanky, with congenitally low blood pressure. There's just too great a distance for blood to travel between my heart and brain. "A natural 'g-monster,' " Dr. DeLorenzo says, "will often be short, with blood pressure on the high side." Those with greater cardiovascular fitness also have problems pulling high g's because of lower heart rates. As a countermeasure, pilots do a lot of strength training.

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The doctor and I are sitting in his office, down the hall from the centrifuge. "Your nausea resulted from the spatial disorientation," he says. "You felt as if you were continuing to spin even after you'd come to a stop. Felt like you were somersaulting forward, right?" I nod. As the doctor explains it, deep within each ear is a set of three semicircular canals, which detect angular motions, and two otolithic organs that sense gravity and linear accelerations. Without the otoliths, fluid-filled sacs containing a bed of microscopic hairs with tiny stones, or otoconia, resting on them, you'd constantly fall down. When you turn your head, the fluid in the canals sloshes the other way, and a separate set of hairs senses this. When you accelerate in one direction or look up at the sky, the otoconia exert a shearing force on their hairs. (Imagine pushing your hand forward quickly while holding coins in an open palm. The coins slide backward and pile up.) "Your eyes tell you you've stopped," says Dr. DeLorenzo, "but the canals and otoconia are still sending the sensation of motion to the brain."

Likewise, when you're trying to read in the backseat of a moving car, your inner ear tells your brain that you're in motion, but your eyes--fixed on the words on the page--don't register the movement. The result: nausea.

Another jolt. A loud click. The g suit inflates. When they tested it on me in the ready room, I felt as if a boa constrictor were squeezing the life out of my lower extremities. Here, at 3 g's, I don't even notice the pressure. One problem, however: I panic, exhaling too much air on my initial breath. I recognize it right away and try to correct, but I can't. When we decelerate, again feeling nauseated, I apologize.

"But you realized it quickly and tried to correct. That's what counts." Johnson's voice has the tone of a special-education teacher. "Now we're taking you up to 5 g's in less than a second. So--this is important--you have to be very, very stingy on that first air exchange. Okay? Ready? One, two, three . . . engage."

I lose the green lights immediately. Midway through the 15-second run, I lose about 50 percent of the red light. The weight on my chest, if possible, bores even heavier this time. I feel as if my nose has flattened, and the skin covering my face has been pulled back to where my ears should be. I must look as yanked as Joan Rivers.

When we stop, my body is aching. There's a scientific explanation for this, too. It's called sopite syndrome--the drowsiness, fatigue, and apathy we feel after a day of flying.

"We blame it on disrupted eating patternsand sleeping habits," says Eric R. Muth, Ph.D., an associate professor at Clemson University who's studied human stress and motion science. "But it's partly the result of all the g-force changes the body goes through." One way to combat motion sickness, Muth goes on, is to drink lots of water. "The vestibular system has fluid in it," he explains. "If you're dehydrated, you get nauseated because the fluid balance of the inner ear is not optimal."

A video simulation pops up. Some kind of aerial dogfight. I suck at video games. A red-and-white circle appears on the screen. It resembles a tiny beach ball. "That's your bogey," Johnson says.

He explains that this time I'll be managing my own speed, using the control stick to my right. Up in the control room, they'll lock the centrifuge at a 5-g ceiling. Chasing that bogey, I can bring myself up to 5 g's anytime by pulling back hard on the stick. He suggests I practice before we get started. I crash my fighter twice, but they close the gondola door anyway.

Johnson: "Whenever you're ready."

I accelerate slowly, tepidly chasing that blasted bogey. Easy now. Two g's. Two and a half. Hard left. Lower body tight, 3 g's. Four g's. Back down to 3. I practice my breathing before taking her up. Five g's. One, two, three, "hoo-kehh-eh." The bogey spirals. I crash.

Mercifully, Dr. DeLorenzo intervenes. We're done. "Your body's going to feel like it's been in a washing machine, probably for the rest of the day," he warns. "Maybe tomorrow, too."

Before I leave, he hands me a videotape of all my runs. I watch it that night in my hotel room, my arms besieged by "geasles"--a rashlike phenomenon caused by blood rushing back into the extremities. During the g runs, I look like the sorriest pencil-neck ever allowed in a cockpit. Between runs, I wear the expression of a terrified chipmunk. The tape will go to my grave. As will the inclination to ever again read in a moving automobile.

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